Conventional incandescent bulbs mostly include a conductive filament, such as a tungsten filament, supported by lead frames which are connected to an external power source via a bulb base to supply electricity to the filament. The filament is rendered incandescent by current flowing therethrough and thus generates light that radiates outward uniformly and extensively. The conventional incandescent bulb, though capable of a wide lighting angle, is disadvantageous because of its high power consumption, high temperature, and short lifetime. By contrast, a light-emitting diode (LED) bulb has a long lifetime, is power saving, produces no wastes that may cause pollution, and is therefore environmentally friendly. Hence, LED bulbs are gradually replacing the conventional incandescent bulbs and are regarded as the new generation lighting devices. However, the limited lighting angle and high production costs of LED bulbs have restricted their applicability in our daily lives.
U.S. Patent Application Publication No. 2005/0254264 discloses an LED bulb which includes a bent circuit board mounted with LEDs thereon, to provide more extensive and uniform illumination in a three-dimensional space by arranging each of the LEDs to have a light-emitting direction perpendicular to the bent circuit board. However, this LED bulb still has its drawbacks such as high production costs, difficult assembly, and a hard-to-control yield. In addition, a wide lighting angle is unattainable if fewer LEDs are used. Moreover, to expose heat radiating ribs, the circuit board cannot enclose the lateral sides and thus, there will be no LEDs at the lateral sides. Consequently, the LED bulb cannot provide effective lateral illumination.
On the other hand, while it is common practice to connect several through-hole LEDs together for multi-angle light emission, the slender pins typical of commercially available through-hole LEDs tend to cause lack of stability and reliability in the resultant mechanical structure. The multi-angle illumination is achieved by bending the pins of LEDs to different directions, and thus the overall structural stability of the finished product will be even lower. The connection between the pins of LEDs may also be problematic. For instance, short circuit and safety hazards may arise from improper arrangement or spacing between the pins when they are electrically conducted.
Taiwan Pat. No. M340562 provides a lighting device which includes top-emitting LEDs mounted on the central region of the top surface of a circuit board to provide illumination to the front side of the circuit board, side-emitting LEDs mounted on the peripheral region of the top surface to provide illumination to the lateral side of the circuit board, and driver circuitry for driving the LEDs is mounted on the bottom surface of the circuit board. Since all the LEDs are disposed on the top surface of the circuit board, they do not provide illumination to the backside of the circuit board. Furthermore, the LEDs and the driver circuitry for driving the LEDs are mounted on the opposite surfaces of the same circuit board, and thus gather heat within a small area. As a result, it is hard to provide effective heat dissipation for the circuit board and the elements mounted thereon, and overheating is likely to occur, thereby shortening the lifetime and impairing the reliability of the lighting device.
Therefore, it is desired an LED bulb which has a wide lighting angle and multiple light-emitting directions, can effectively dissipate heat so as to maintain the lifetime of the LEDs thereof, is reliable in terms of structure and design, and incurs low production costs.